SCR systems used in heavy duty vehicles such as, for example, off-highway trucks and on-highway buses typically use ammonia as a reducing agent which is injected into the exhaust in the form of aqueous urea. At unsuitable operating conditions, this urea can form solid deposits in the exhaust instead of the desired production of ammonia, and over time these deposits can block the exhaust and require maintenance. Avoiding deposits is a complicated process, because they are difficult to predict and to detect without a visual inspection.
US2013-0192206A1 proposes an approach where deposits are detected by the ammonia released during the thermal decomposition of deposits at high temperatures. However, the approach is indirect, has limited accuracy and is not helpful during long periods of low temperature exhaust flow when deposits are most likely. An alternative proposal has been made to measure the pressure drop across the exhaust section, but it is not sensitive enough to detect deposits early on as a significant pressure drop is only formed once the exhaust is nearly blocked.
It is an aim of the present invention to obviate or mitigate the disadvantages of the aforementioned proposals.